SAINTOPIN, A DUAL INHIBITOR OF DNA TOPOISOMERASE-I AND TOPOISOMERASE-II, AS A PROBE FOR DRUG-ENZYME INTERACTIONS

Stabilization of the topoisomerase-cleavable complexes is the common i nitial event leading to the cytotoxicity of topoisomerase I and II (to p1 and top2) inhibitors. Using saintopin (STP), a poison of both topoi somerases, we studied top1- and top2-cleavable complexes (Yamashita, Y ., Kawada, S.-Z., Fujii, N., and Nakano, H. (1991) Biochemistry 30, 58 38-5845). top1 and top2 sites induced in the presence of STP showed th e same preferences for the base located 3' to the topoisomerase-induce d DNA break (position +1): preference for G and not C. A camptothecin- resistant top1 with a mutation (Asn(722) --> Ser) next to the catalyti c tyrosine (Tyr(723)) was cross-resistant to STP, suggesting that both STP and camptothecin interact with the protein near the catalytic tyr osine. These results are consistent with a dual interaction of the dru g with the enzyme and the DNA and provide further evidence for the ''d rug-stacking'' model. This model proposes that topoisomerase inhibitor s bind, possibly through hydrogen bonding and/or stacking, with one of the bases flanking the DNA termini (guanine at position +1 in the cas e of STP) and within the enzyme catalytic pocket, most likely by stack ing with the catalytic tyrosine.